As developing countries begin to prosper, fossil fuels will continue to be the primary source of the world's energy. Large deposits of coal throughout the world and the United States are relatively inexpensive and geopolitically stable. The environmental costs of burning coal, however, can be large. Carbon dioxide, which is formed in the conversion of stored chemical energy in fossil fuels to electrical energy, is generally accepted to be a “greenhouse” gas and a major contributor to global warming. Energy efficient capture and/or conversion of carbon dioxide is therefore an important technical challenge. Carbon dioxide is an extremely stable compound at standard conditions, and a large amount of energy is required to reduce it.
Titanium dioxide is a large band-gap semiconductor, which absorbs ultraviolet radiation to form an electron-hole pair. The solar-generated charge carrier can be transferred to adsorbed surface molecules, such as water or carbon dioxide, to promote oxidation and reduction reactions, for example reduction of carbon dioxide to methane or other hydrocarbons (e.g., ethane). To date, the photocatalytic reduction of carbon dioxide is highly inefficient due to limitations in the solar spectrum absorption, electron-hole recombination events, and charge transfer of uncombined electron-hole pairs to adsorbed molecular species on the titanium oxide surface.
In recent years, effort has been made to improve the efficiency of photocatalytic reduction of carbon dioxide (also referred to herein as “solar fuel conversion”). Cocatalyst structures consisting of a combination of metal oxide semiconductors and noble metals, and highly doped metal oxide semiconductors have been proposed in order to “tune” the adsorption edge (i.e., bulk band gap) of the photocatalysts. These approaches reportedly have increased the conversion efficiency to some degree; however, inefficient conversion has remained a significant impediment to widespread commercialization of this technology.
Consequently, there is an ongoing need for improved catalysts for photochemical (particularly solar) reduction of carbon dioxide to hydrocarbon fuels such as methane. The present invention addresses this ongoing need.